My husband played guitar for 10 or so years without "schooling" himself on theory. He understood the concepts and fundamentals but never got into the details and the why's. In the past 6 or so years he's done much study and it has connected SO many dots for him and expanded the neck and frets into things he feels stupid for not having seen before - he saw the paterns, but now he sees the designs in the patterns and has come up with his own teaching method.
We know some people whom my husband is recording. At first they said they don't need to know theory, it put them in a box so they can't explore their art. They were 2 songwriters, a guitarist ho wrote the music and a vocalist who wrote the lyrics. Well, as the lyrics were getting written she'd direct the guitarist to go up with her at certain parts. Well this led to songs with 8-10 chord changes, no even measures - even the verses had different measures from the other verses - and left the vocals "stuck" singin everything the same way every time. The guitarists timing was off because every chord change had to corrispond with the vocals, and she'd even forget the songs because of the amount of chord changes. You couldn't tell the verse from the chorus because they were already using EVERY note in the scale twice! Our drummer (who has over 30 years experience) came in as a studio drummer began to chart out the songs, when he finally finished he was like...."ummm...okay, this is gonna be fun". Since then we've helped them rework the songs starting with the guitarist, taking out unecessary notes, simplifying and diversifying the strum patterns, adding breaks, etc. Then the singer had to figure out how to sing them. She didn't have to change much and found she had MUCH more freedom. Theory saved the day, and they are glad to be out of the box they HAD been in before they realized NOT knowing theory was the constraint.
Even as a vocalist I want to know theory, I want to know where my 3rds, 5ths and such are and what scale I should be singing in for a particular song so I can compliment the music and become part of it - not my vocals over some sounds. I AM another instrument and it is just as important for me as any other musician.